Field of the Invention
Pollution control, particularly methods for quantification of airborne fugitive emissions. Traditionally, such fugitive emissions are measured by one or more of the following techniques:
1. The quasi-stack method involves the installation of a hood over an isolatable source so that the pollutant is transmitted through a duct of known cross-section thereby permitting application of standard stack sampling (confined source) techniques involving measurement of flow rate and pollutant concentration. This approach therefore involves conversion of an element of fugitive emission into a confined emission but is seriously limited since it is not a practical approach for the case of a multiplicity of emission points or for large sources; or for disseminated area sources; or for complex, variable and ill-defined operations, as described above.
2. The roof monitor method is applicable to pollutants generated within a building which enter the atomsphere through an opening in the building. This is a version of stack sampling (confined emission) where the opening is so large that pollutant concentrations and flow rates must be measured at a number of points over the plane of the opening and then integrated to obtain the total emission rate into the atmosphere. This approach is applicable only to the case of pollutants generated indoors, and where there is a large but well-defined opening to the atmosphere. It is not useful for a multiplicity of emission points, for disseminated sources, or for complex, variable and ill-defined operations as described above.
3. The upwind-downwind method is based on measuring atmospheric concentrations downwind of the emission source, as well as upwind, and attributing the difference to the source. The critical element in this technique is the use of mathematical models of atomspheric diffusion along with meteorological measurements to back-calculate source strength. Of the three general methods, this is the only one which in principle has any capability for dealing with the case of unconfined sources. The limitations on the method, however, are significant. First, it must be presumed that the atmospheric diffusion model is accurate in describing behavior of pollutants after release into the atmosphere. Generally speaking, even for the simplest case of a ground level, continuous point source, the agreement between model and observation is only approximate. Second, the method is incapable of treating the more common problems represented by emission sources of arbitrary and irregular geometry, including multiple elements at various heights. This is so because diffusion formulae characteristically are available only for simple source geometries such as point, line, uniform area, or normally distributed sources. The practical problem to be confronted, however, is that of complex, multiple, non-uniform and unconfined pollution sources. Particularly, the absence of prior knowledge of the spatial distribution of pollution source strength elements, which might otherwise permit mathematical integration of the simple diffusion formulae, -- prohibits quantification of fugitive emissions. Spatial distribution of source strength, of course, is one of the very objectives which fugitive source evaluation seeks to measure.
In summary, none of the available methods is capable of providing a direct measurement of pollution emission rates for the case of unconfined, fugitive sources of arbitrary and complex geometry.